Oncornaviruses exist as endogenous genetic elements in the cells of many, if not all, animal species. As such, the expression of these genes is obviously under the control of the host cell and may well occur as a normal function of the cell. The object of the proposed research is the analysis of the structure, organization and control of expression of these genes. Two experimental systems, mouse genetics in vivo and somatic cell hybrids in vitro, will be employed; (1) Mechanisms of host cell control in vivo of the synthesis or metabolism of viral gene products will be analyzed by quantitative measurement of specific type-c virus proteins in tissues of different strains of mice and their offspring, (2) Rodent-human somatic cell hybrids will be utilized to study the organization of viral genes, their chromosomal site of integration, and mechanism for control of gene expression. The techniques employed will be a combination of the genetic and biochemical, based upon the quantitative radioimmunoassay measurement of specific viral proteins.